This invention relates to refrigeration systems, heat engines and other thermodynamic apparatus which usually utilize a piston in a cylinder for compression of a gas. Sometimes, bellows are used in place of pistons because they avoid the life-limiting wear between piston and cylinder, they eliminate the contamination of any oil required for piston lubrication and they operate without the usual type of friction and leakage. It could be argued that only bellows seals offer an opportunity of achieving theoretically infinite life in such apparatus. But bellows have disadvantages.
In terms of disadvantages, there are two types of bellows, the formed bellows and the welded bellows. This invention applies to the formed type. Formed type metal bellows are usually made from either a welded or a seamless thin-walled tube, preferably the latter. Neglecting electroforming, the forming process usually involves a combination of rolling, squeezing and the use of hydraulic pressure. Their convolutions are characterized by smooth sections such as those of torroids; such cross sections are often identified as "U-shaped", "S-shaped", etc. See ASME Publication PVP-Vol. 83, page 24 which is hereby incorporated herein by reference. The main disadvantage found in formed types of bellows is their inability to be safely compressed to a mode shape characterized by small dead volume. Dead volume is the clearance volume which remains inside of a bellows when the bellows is fully compressed. Some applications require that the dead volume be less than one tenth of the total displacement volume. Most formed type bellows can not meet this requirement.
The second of the two major types of bellows is the welded bellows. This refers to various welded disk types wherein an extensive amount of welding is required in fabrication. It does not refer to a formed bellows made from a welded tube. The main disadvantage of welded bellows is a tendency toward high stresses and toward greater uncertainty when calculating such stresses. To reduce these disadvantages, various methods of obtaining tight control of the welding process have been tried. However, so far, the results have quite often been disappointing when compared to the reliability of welds in other types of structures. The reasons include the coincidence of the weld and the maximum stress location resulting from this geometry, problems with double welding, the sharp notch effect, an inability to clean and inspect welds from inside, the exposure to stress concentration over a large total length of welds, etc. For these reasons, welded bellows are not suited to meet very long life applications.
In this type application, what is needed is a bellows that can not only improve system efficiency by having less dead volume but also improve reliability by offering the designer a chance to lower stresses as if dead volume could be ignored. Page 98 of the EJMA Standards shows the reliability benefit from reduced stresses. To reduce both stresses and dead volume simultaneously, a variety of rather simple design changes have been tried with limited success. Examples are the shape shown in FIG. 6 of U.S. Pat. No. 3,469,502, reinforcing rings of U.S. Pat. Nos. 2,323,985 and 3,135,295, etc. What is needed now is an approach to the design of bellows that promises far larger reductions in stress and dead volume than these simple changes can provide.